This invention relates to light sensors, and more particularly to photosensitive devices utilizing a photovoltage effect, such as photodiodes.
Silicon and germanium photodiodes have been known. Such conventional photodiodes have either a PN junction formed by an impurity diffused region or a surface barrier formed by metal-to-semiconductor contacts. When photodiodes are exposed to a light, some pairs of electrons and holes are produced causing current to flow through the PN junction or the surface barrier. This is called the photovoltage effect.
The photo-sensitivity of such photodiodes lies within a spectrum ranging from visible to near-infrared wavelengths. Therefore, to enable this conventional photodiode to be sensitive only to the visible spectrum of light, an infrared filter capable of absorbing radiation having wavelengths longer than about 750 nm has had to be placed in front of the photodiode. Typically, this filter comprises either an infrared absorbing glass such as borosilicate glass which includes metal ions such as Fe.sub.3.sup.+ and/or the like, or an infrared reflecting filter constituted of a glass substrate and multi layers piled up alternately on such substrate comprising two kind of interference films, one of which is a high refractive index material such as ZnS and the other of which is a low refractive index material such as MgF, or the filter may be a combination of the above two filters. Such filter is indispensable if the conventional photodiode is to be applied to a camera for automatic control of exposure. With this filter, however, it has been virtually impossible to obtain a stable infrared filtering effect and visible light transmission characteristics and to provide a photodiode of of reduced cost. Furthermore, the use of such a filter has resulted in an undesirable addition to the size of the photodiodes.
Another drawback involved in the conventional photodiode is its inability to measure dim light because the light-to-dark current ratio of such a device is less than 600 for illumination of one lux. or under. The above has been derived from experimental results which demonstrated that the leakage current (or dark current) which flowed when a reverse bias-voltage of 2 to 3 V was applied to a conventional photodiode in the absence of light at room temperature was larger than 200 PA/cm.sup.2, while the light current which flowed in such a device under illumination of one lux from a light source having a spectrum equal to the radiation spectrum of a black-body of 2854.degree. K. was 120 nA/cm.sup.2 at most.
Therefore, it is a principal object of the invention to provide a light sensor which is sensitive only to a specific spectrum of radiation, has uniform spectral sensitivity, can be constructed in a small size and can be manufactured at low costs.
It is another object of the invention to provide a light sensor capable of markedly reducing the dark current and thus offering a greater light-to-dark current ratio.
It is still another object of the invention to provide a light sensor sensitive to visible light or to light having a wavelength of from 350 to 750 nm.